Repairing the damaged plastics of a motorcycle is one thing; painting them to a satisfactory level is an altogether different matter. Unless you have the facilities to properly spray it’s arguably a job that is best left to the professionals.
However, should you wish to take the plunge and have a go there are almost limitless tutorials online that will take you through preparations, operating conditions, paint types and numerous other key areas. One thing to be aware of with plastics is getting the right primer; without this vital first step, paint is unlikely to adhere to the plastic properly.
The hardest plastic to cut in terms of paint adherences is anything regarded as a polyolefin. If the panels you’re working on look and feel like a washing up bowl then they’re almost certainly polyolefin in nature. Polyethylene (PE) and polypropylene (PP) are the most common and are typically found on step-through leg shields and mudguards.
Special primers exist specially for materials such as these and their use should be regarded as mandatory if the paint is to stay where it’s sprayed. If the panels being painted are known to be prone to flexing it’s common practice to use a paint that’s been modified with a plasticiser which ensures the resin used remain flexible.
Like any other maintenance and DIY job that’s outside your skill set, if you don’t know always ask an expert.